I am only bringing this up because this game is free and pretty awesome. You might have heard of it before. It was made from Japan by "Doukutsu". If you download the deluxe version it has everything translated to English.
Easiest way to explain this game is it a full screen side scroller. It plays like metroid, zelda 2 and castlevania ds games. There is a lot of RPG aspects to it and most NPCs you have to talk to before you can get to a certain part of the game. On top of that it looks like an NES game.
It works on windows, mac, linux, psp homebrew, xbox one, even a port to a TI-83/84. I have heard it also soon might be a downloadable game on Wii.
Go play it. This game is very addicting.
http://www.miraigamer.net/cavestory/